internal/jsonrpc2_v2: eliminate error return from Bind

Also make (*Connection).Close safe to call from Bind.

A jsonrpc2_v2.Server has no good way to report an error from Bind.

If the Server saves the error to return from its own Wait method, that
might not ever actually happen: Wait waits for in-flight connections
to complete, but if some existing connection stays up then Wait will
not return.

If the Server goes ahead with establishing the connection and installs
its own Handler, that Handler needs to decide whether to serve the
error from Bind or something more opaque, and at that point Bind may
as well return a handler that makes that choice more precisely.

If the Server merely logs the error and closes the Connection, then
the Bind method itself may as well do that directly too.

It seems to me that the only winning move is not to play. Only Bind is
in a position to decide how to handle its errors appropriately, so it
should not return them to the Server.

Updates golang/go#56281.

Change-Id: I07dc43ddf31253ce23da21a92d2b6c0f8d4b3afe
Reviewed-on: https://go-review.googlesource.com/c/tools/+/443677
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

internal/jsonrpc2/jsonrpc2_test.go

@@ -253,7 +253,7 @@ func verifyResults(t *testing.T, method string, results interface{}, expect inte
 	}
 }
 
-func (b binder) Bind(ctx context.Context, conn *jsonrpc2.Connection) (jsonrpc2.ConnectionOptions, error) {
+func (b binder) Bind(ctx context.Context, conn *jsonrpc2.Connection) jsonrpc2.ConnectionOptions {
 	h := &handler{
 		conn:    conn,
 		waiters: make(chan map[string]chan struct{}, 1),
@@ -267,7 +267,7 @@ func (b binder) Bind(ctx context.Context, conn *jsonrpc2.Connection) (jsonrpc2.C
 		Framer:    b.framer,
 		Preempter: h,
 		Handler:   h,
-	}, nil
+	}
 }
 
 func (h *handler) waiter(name string) chan struct{} {